The present invention relates to converter circuits and, more particularly, to a circuit for converting a positive supply voltage to a negative output voltage.
The art is replete with +DC to -DC voltage converter circuits for generating a negative output DC voltage from a positive supply voltage. There are a myriad of systems applications for such converter circuits. For instance, many computer systems include a keyboard that is peripheral to the mainframe and are coupled to the central processing unit (CPU) through a coiled connecting cord. Usually the keyboard electronics is Transistor to Transistor Logic (TTL) compatible, with a high logic level of approximately 2.4 volts and a low logic level of 0.4 volts providing a two volt noise margin. Generally however, when the data from the keyboard electronics is interfaced to the CPU via the connecting cable a higher noise margin is desired wherein the TTL logic level swing is converted to a larger voltage swing such as the RS423 standard. The RS423 standard provides a minimum of 7.2 volts of noise margin with a single ended driver output swing of +/-3.6 volts. Because it is desirable to operate the keyboard electronics from a positive supply voltage a converter circuit is required to provide the negative voltage for the low level of the RS423 output.
Hence, a need exists for a simple, efficient circuit for converting a positive DC voltage to a regulated negative DC voltage which can be included in a single integrated keyboard system, for example.